The answers are just as clueless. The prime function of railway civil engineering is to keep the line either on a level or on a constant gradient. There are exceptions, usually rail over rail, but it’s not the bridges that determine the height of an embankment or the depth of a cutting, it’s the earthworks and topology of the ground that determine the size of the bridges.

Of course some people don’t model the real railway, they make copies of other people’s models because they’ve never thought about looking at the real thing for inspiration.

It’s not really that hard to make your own bridges, for a fraction of the cost of the resin offerings from the likes of Hornby and Bachmann. When you make your own you can determine the height yourself to suit that of the railway line above ground level, not unlike how real railways are built, but even if using ready made bridges why is this person asking the question? If he has got a bridge already his railway will need to be high enough to cross that bridge… Unless he uses a hacksaw and cuts the abutments down to fit… But if he’s got a hacksaw he’s probably got the other tools and materials* required to make a model so he might as well save twenty or so quid by starting from scratch or using a kit.

* Craft knife (or even scissors) and cardboard for basics.

It is quite amazing how a model railway forum can descend to the level of the lowest common denominator, then keep going until it hits rock bottom.